Venereal syphilis is a sexually transmitted disease caused by the spirochetal pathogen, Treponema pallidum. The disease remains uncontrolled, particularly in resource-scarce regions where testing, treatment and proper follow-up are not always accessible. Syphilis is characterized by the appearance of a local ulcer at the site of initial infection to an array of chronic systemic inflammatory manifestations that when left untreated, can cause severe complications and even death. The inability to culture the bacterium has hampered efforts to fully understand its pathogenesis, particularly why the spirochete is able to evade the immune system. The research proposed here is thus designed to better characterize the nature this immune response as it occurs in human syphilis. The systemic cellular innate immune response in secondary syphilis patients will be the focus of the current proposal. We hypothesize that in secondary syphilis, the presence of T. pallidum in the blood directly activates circulating monocytes and dendritic cells. To do so, in Specific Aim 1 we will first confirm that spirochetal DNA is present in blood from secondary syphilis patients using a novel Real Time Quantitative PCR method and in Specific Aim 2 we will characterize T. pallidum driven immune responses in circulating monocytes and dendritic cells by using multiparameter four color flow cytometry and `real time'PCR (RT-PCR) to quantify their pro- inflammatory cytokine gene expression signals. Complementary ex vivo studies, are structured to identify the effect of the bacterium on specific components of innate and adaptive immune cells. From these two parallel lines of research, we will obtain a more mechanistic understanding of the pathogenesis of syphilis, which can be further developed along cellular and molecular lines and ultimately facilitate development of a vaccine. In concert with the Fogarty International Center's mission to reduce health disparities amongst nations, the planned activities will also: (1) support collaborative syphilis immunobiology research between the USPI and the FC in Cali, Colombia;(2) strengthen the foreign institute's (CIDEIM), research capabilities and technical base;(3) foster sustained and productive research between two US based institutions (UCHC and CCMC) and CIDEIM;and (4) provide needed epidemiological data about venereal syphilis for that region of Colombia.